Save a word in the info table on x86_64

Summary:
An info table with an SRT normally looks like this:
StgWord64 srt_offset
StgClosureInfo layout
StgWord32 layout
StgWord32 has_srt
But we only need 32 bits for srt_offset on x86_64, because the small
memory model requires that code segments are at most 2GB. So we can
optimise this to
StgClosureInfo layout
StgWord32 layout
StgWord32 srt_offset
saving a word. We can tell whether the info table has an SRT or not,
because zero is not a valid srt_offset, so zero still indicates that
there's no SRT.
Test Plan:
* validate
* For results, see D4632.
Reviewers: bgamari, niteria, osa1, erikd
Subscribers: thomie, carter
Differential Revision: https://phabricator.haskell.org/D4634